Lubricant oils are used for internal combustion engines, automatic and manual transmissions, differential gears, power steerings, shock absorbers, and gears, for their smooth operation. In internal combustion engines, in particular, lubricant oils have been used mainly for piston rings, cylinder liners, bearings for crank shafts and connecting rods, valve trains including cams and valve lifters, and other sliding members. In addition to the lubricating purposes above described, they are also used for cooling engines, cleaning and dispersing combustion products, and prevention of rust and corrosion.
As described above, lubricant oils for internal combustion engines are required to exhibit a variety of functions. These requirements are becoming even severer, as the engines become more functional, produce higher power and are operated under severer conditions. In order to satisfy these requirements, base oils for lubricant oils for internal combustion engines are incorporated with a variety of additives, such as ashless dispersants, metallic detergents, antiwear agents, friction reducing agents and antioxidants.
Combustion gases produced by an internal combustion engine partly leak into the crank case as blow-by gases through a space between the piston and cylinder. NOx gases contained in the combustion gases at a fairly high proportion can deteriorate a lubricant oil in the internal combustion engine, in a concerted manner with oxygen present in the blow-by gases. Lean-burn engines are now being widely used, to improve fuel economy. These engines are equipped with a three-way catalyst system to prevent/reduce NOx or exhaust gas recirculation (EGR) system to abate NOx emissions. A three-element catalyst is known to be poisoned by sulfur, and it is necessary, when the catalytic system is used, to control the sulfur poisoning resulting from evaporation of the engine oil. It is also necessary, when an EGR system is used, to control deposit at the intake valve and contamination of the EGR control valve with the engine oil components, resulting from inflow of the engine oil into the EGR system.
An engine oil for internal combustion engines, in particular lean-burn engines, is required to be low in volatility and difficult to be deposited even when it is evaporated to flow into the EGR system. In other words, it is required to be high in oxidation stability. Deposits can be also formed by sludge in the oil, resulting from oxidation and deterioration of the oil by NOx present in the blow-by gases, and the oil is required to control formation of such sludge.
A variety of additives have been proposed to improve oxidation stability and serviceability of engine oils for internal combustion engines. These engine oils include solid impurity agglomerating type diesel engine oils incorporated with a calcium phenate, magnesium sulfonate and alkenyl succinimide (Japanese Patent Publication No.3-29839), diesel engine oils incorporated with a combination of an ashless dispersant, metallic detergent and the like (Japanese Patent Publication No.6-60317), engine oils incorporated with an antioxidant of sulfur-containing phenol derivative or the like (Japanese Laid-open Patent Application No.6-93281, equivalent to U.S. Pat. No. 5,569,405), engine oils incorporated with a specific antioxidant or the like (Japanese Laid-open Patent Application No.7-126681, Derwent Abs. 95-213285/28), and diesel engine oils incorporated with a combination of 3 types of additives (Japanese Laid-open Patent Application No.7-207290, Derwent Abs. 95-309399/40).
A variety types of base oils have been also proposed to improve properties of engine oils. These base oils include the one based on mineral oil prepared to have a viscosity index of at least 80, and contain basic nitrogen at 5 ppm or less and aromatic compounds at 1% or less for the lubricant oil composition serviceable in a NOx-containing atmosphere (JP 2,564,556, Derwent Abs. 89-050214/07), the one based on mineral oil or the like prepared to have a viscosity of 2 to 50 cSt at 100.degree. C. and contain aromatic compounds at 2% or less for internal combustion engine oils serviceable in a NOx-containing gas atmosphere (Japanese Patent Publication No. 6-62988), and the one based on mineral oil containing total aromatic compounds at 2 to 15 wt. %, and isoparaffin and monocyclic naphthene compounds at 60 wt. % or more as total content in the saturates (JP 2,724,508, equivalent to U.S. Pat No. 5,372,703).
It is accepted that use of a base oil consuming less oil or engine oil incorporated with a lower content of viscosity index improver is an effective means to control or prevents deposit on intake valves or the like for lubricant oils for internal combustion engines.
In spite of these proposals, however, no lubricant oil composition can sufficiently control poisoning of a three-way catalyst system for preventing/reducing NOx and deposit in an air intake system in lean-burn engines.
In an embodiment, the present invention provides a lubricant oil composition for internal combustion engines, excellent in resistance to oxidation in the presence of NOx and evaporation characteristics, and controlling poisoning of a 3-element catalytic system for preventing/reducing NOx and formation of deposit in an air intake system.